1. Field of the Invention
The present invention relates to improvement in a single focus imaging lens for use in an optical system of various cameras such as silver-salf camera, digital camera, video camera, security camera. In particular, it relates to an imaging lens suitable for an imaging device incorporating an electric imaging element such as a digital camera or video camera as well as a camera and a hand-held data terminal device incorporating such an imaging lens.
2. Description of the Related Art
A digital still camera or digital video camera including a solid image sensor as CCD, CMOS is widespread. Especially, a still-image digital camera has replaced a silver salt camera. In a developed digital camera market users' demands are various and diverse. Users place great expectations on high-quality compact-size cameras incorporating a large image sensor with a width across corners of 20 mm to 45 mm and a high-performance single focus lens. Users expect a higher-performance, smaller-size, and higher-portability digital camera. In terms of high performance, such a camera needs to exert resolution equivalent to an image sensor with 10 to 20 million pixels, generate, at the maximal aperture of a diaphragm, images at a high contrast, with less coma flares, less blurs in point images even at the periphery of angle of view, less color aberration and no false color even in a portion with a large brightness difference, and less distortion. In view of a larger diameter, it requires at least F2.8 or less for the purpose of differentiating from a general compact camera mounting a zoom lens.
In view of downsizing, an actual focal length is long due to the use of a relatively large image sensor so that the total lens length standardized by a focal length or maximal image height has to be shorter than when a small image sensor is used. Further, a half angle of view is preferably 28 degrees or more with users' preference to a wide-angle lens taken into account. A half angle of view of 28 degrees is equivalent to a focal length of about 41 mm of a 35 mm silver salt camera.
A typical wide-angle, single focus lens is a retrofocus lens configured of a lens group with a negative refractive power on an object side and a lens group with a positive refractive power on an image side. The retrofocus lens can meet the demand that exit pupil position is set far from the image plane and periphery light beams should be incident on an area sensor at an almost vertical angle due to the property of the area sensor having a color filter and a micro lens on each pixel. However, the retrofocus lens originally aims to be used as a replacement lens of a single lens reflex camera and secure a back focus so that the total lens length from an object-lens surface to the image plane tends to be long.
Meanwhile, there has been progress in relatively large images sensors with width across corners of 20 to 45 mm owing to improvement or optimization of on-chip micro lenses and advanced image processing. Therefore, a diagonal incidence of periphery light beams on the sensor does not cause a large problem. For example, an optical system which allows an angle of 30 degrees or so between a principal beam and the optical axis at a maximal image height can be designed. Thus, the vertical incidence of peripheral beams does not matter much and a lens suitable for a downsized device is now available.
A symmetric type lens or a telephoto lens including a lens group with a negative refractive power on image side is more appropriate for a compact camera. Such a lens is disclosed in Japanese Patent Application Publication No. S63-24213 (Reference 1), No. H09-236746 (Reference 2), No. 2000-321490 (Reference 3), No 2005-352060 (Reference 4), and No. 2009-216858 (Reference 5), for example.
The imaging lens disclosed in Reference 1 is a telephoto lens of four groups and four lenses often used in a silver film compact camera. It is a small size but insufficient in terms of optical performance due to curvature of field and large astigmatism. The imaging lenses in References 2 and 4 are of a small size with high imaging performance but not good for a digital camera since the angle between the principal beam and optical axis exceeds 35 degrees at the maximal image height. The imaging lens of a large diameter in Reference 3 includes a large number of lenses and is not cost effective. The imaging lens in Reference 5 faces a problem in downsizing because of a large total length relative to focal length. Further, the imaging lenses in References 3 to 5 has a problem with downsizing and portability since it does not include a wide air space and secure a back focus and cannot be contained thinly even with use of a collapse mechanism.